This invention relates to an improved electrodeionization system in which the power supply is pulsed on and off to maintain output resistivity in a predetermined range.
Electrodeionization systems are used to remove ions from liquids, especially water. These systems require a power supply that applies voltage to the electrodeionization module. Typically, these power supplies are constant DC voltage power supplies, in which the current is varied to maintain a constant voltage. Another option is to use a constant DC current power supply, in which the voltage is varied as necessary to maintain a constant current.
The output liquid quality, measured in terms of resistivity, is in part dependent upon the input water quality, and in part dependent on the voltage applied to the electrodeionization module. Because of differences in the feed liquid ion composition and concentration, constant voltage power supplies need to be sized with a sufficient output current to handle feeds that have high ion concentration, especially water that contains high concentrations of weakly ionized species such as carbon dioxide and silica. Constant current power supplies suffer from the same drawback. Additionally, if the feed water has a lower ion concentration, especially of carbon dioxide or silica, such power supplies are often larger and more expensive than is needed to accomplish the result. Also in such cases, the electrodeionization module is continuously subjected to a higher voltage (or current) than necessary, which may contribute to poor performance as well as shortening the useful life of the module. Further, scaling of the electrodeionization module can be a problem when there is more current than necessary applied to the module.
It is therefore an object of this invention to provide an improved electrodeionization system that can use a more efficient power supply.
It is a further object of this invention to provide such a system that lowers power consumption, especially on low ion concentration feed waters in the order of 30-50%.
Is a further object of this invention to provide such a system that allows for a target range of output liquid resistivity.
It is a further object of this invention to provide such a system which may decrease scaling occurring in the electrodeionization module.
This invention results from the realization that a desired output water quality range can be realized in an electrodeionization system by pulsing the power supply on and off at a duty cycle proportional to the output water quality within the range. This allows the use of a simple, inexpensive constant current or constant voltage power supply, to achieve a tailored quality result.
This invention features an improved electrodeionization system for removing ions from liquids passed therethrough, comprising: a flow-through electrodeionization module; a power supply electrically connected to the electrodeionization module so as to establish a voltage gradient thereacross; first means for monitoring the value (e.g. resistivity) of a property of the liquid output flow from the electrodeionization module; and means, responsive to the first means for monitoring, for controlling the amount of time that the power supply is turned on.
The power supply may be a constant current or a constant voltage power supply. The system may also include second means for monitoring the value of the property of the liquid inlet flow to the electrodeionization module, and the means for controlling may be further responsive to the second means for monitoring.
The means for controlling may comprise a proportional band control means for controlling the amount of time that the power supply is on proportionally within a range of values of the measured property. The system may then further include means for establishing the range of values.
In another embodiment this invention features an improved electrodeionization system for removing ions from liquids passed therethrough, comprising: a flow-through electrodeionization module having a liquid inlet and a liquid outlet; a power supply electrically connected to the electrodes of the electrodeionization module so as to establish a voltage gradient thereacross; a resistivity monitor for monitoring the resistivity of the liquid output flow from the electrodeionization module; means for establishing a target resistivity range for the liquid output flow; and a proportional band control means, responsive to the resistivity monitor and the target resistivity range, for proportionally controlling the amount of time that the power supply is turned on such that the power supply is continuously on when the resistivity is below the range, and is continuously off when the resistivity is above the range, and is intermittently on when the resistivity is within the range.
The control means may establish a power supply cycle time. When the resistivity is within the target range, the control means may turn on the power supply for a portion of each cycle time that is inversely proportional to the monitored resistivity relative to the range.